1. Field of the Invention
The present invention relates to a method for producing coated hot-rolled or cold-rolled products, particularly steel strip. The invention also relates to an apparatus for carrying out the method.
2. Description of the Related Art
For producing hot-rolled and cold-rolled strip preferably of steel in the field of casting methods with approximate final dimensions, the Bessemer two-roller method is known in the art. On the other hand, also known in the art is the inversion casting method (EP-B1-0 311 602; DE C1-43 19 569; DE C1-195 09 681; as well as dissertation of Dr. Vonderbank and Dr. Hamacher/RWTH Aachen, Institut fur Eisenhuittenkunde, 1995).
The Bessemer two-roller method is today used preferably for casting stainless steel but also carbon steel. However, in the case of casting thicknesses of 10 to 2 mm and widths of 500 to 1,800 mm, it is difficult to adhere to a tolerance of the thickness profile over the width of, for example, &lt;1% at the cast product. This desired profile which is required by the market must be adhered to by percentage during casting because, due to the high flux resistance in transverse direction, an improvement of the profile in the roll gap of a roll stand cannot take place.
For practical purposes, it can be stated that, in the case of a width to thickness ratio of &gt;100, a transverse flux in the roll gap of a roll stand does not take place and, thus, a profile improvement cannot be carried out. The production results with the two-roller method and also with other strip casting methods on the basis of wall molds confirm that the thickness deviations in the width direction (profile) as well as in the casting or rolling direction (planarity) are outside of the tolerance for plates required by the steel market.
For example, strip material, preferably of stainless steel, which in the past has been produced on the basis of travelling molds and particularly, for example, on the basis of the Bessemer two-roller method, which up to now is used most widely by numerous strip casting techniques on the basis of travelling molds, can only be utilized for punched products which do not have a high requirement with respect to planarity in the width direction as well as in the rolling direction.
Another strip casting method not based on travelling molds is the inversion casting method. In that case, a thin strip, called master or starting strip, is continuously guided through a melt in a crystallizer over a controlled period of time. Taking into consideration the temperature of the master strip when entering the melt, for example, room temperature or also, for example, 800.degree. C. and the temperature of the melt, expressed as overheating temperature X in .degree. C. (T.sub.actual =T.sub.li +X.degree. C.), during the dwell time of the master strip in the melt a controllable quantity of melt, also called secondary material, crystallizes onto the master strip.
Depending on the above-described parameters, this crystallization of the secondary material onto the master strip, which may have an initial thickness of, for example, between 0.5 to 10 mm, is between 5 and 500%.
Thus, a high percentage crystallization of, for example, 500% occurs onto a cold and thin master strip (e.g. room temperature 20.degree. C. 1 mm thickness), with a low overheating temperature of, for example X=15.degree. C. and a long dwell time of, for example, 2.5 sec. On the other hand, a minimum coating of, for example, 5-10% can be achieved on a master strip preheated to, for example, 600 to 800.degree. C. and with a thickness of, for example, 2 to 5 mm, with a relatively high overheating of, for example, X&gt;15.degree. C. and a short dwell time of, for example 1 sec.
This inversion casting method can be utilized for monomaterials as well as for composite materials and has the significant advantage of providing the possibility of producing a planarity as it is required by the steel market of strips and plates, for example, with a profile &lt;1%, by a smoothing roll pair mounted above the melt or above the crystallizer, as disclosed in DE-C1-43 19 569.
However, when used as a casting method, the inversion casting method requires the partially necessary return of the rolled finished product of, for example, 1%, for maintaining a stock of master strip of 2 days or 4,000 t in the case of an annual casting capacity of 0.6 million t which, in turn, results in an accumulation of defects, for example, oxidic inclusions, in the product which is not acceptable in the long term. This inevitable system-imminent accumulation of defects can only be corrected by using master strip which is purchased from the outside.
Moreover, when coating hot-rolled strip in the range of 50 to 5%, it is necessary to preheat the master strip, for example, to 800.degree. C.
This preheating is technically very complicated when considering that a master strip of, for example, 2 mm is introduced into the melt of the crystallizer with a speed of between 10 and 100 m/min.
Preheating can be carried out, for example, inductively or also with furnace immediately before the master strip enters the crystallizer or also in a shaft within the crystallizer or the melt, as disclosed by DE-C1-195 09 681. This preheating unit must have a high power density, requires a great investment and leads to high operating costs.